How does phylogenetic diversity (PD; Faith 1992) loss increase as the number of species extinctions increases?

Phylogenetic diversity (PD; Faith 1992) is an important target for biodiversity conservation because it represents current and future benefits for humans (evolutionary or “evosystem services”; see Faith et al 2010). Benefits therefore are lost when branches of the phylogenetic tree are lost through species extinctions. Several studies have examined the expected amount of loss of phylogenetic diversity for a given number of species extinctions (reviewed in Morlon et al 2010). The amount of loss depends in part on whether species extinctions are clumped or well-dispersed on the phylogenetic tree. For example, many studies looking at climate change impacts suggest relatively small PD losses, given dispersed climate change impacts over phylogenetic trees (see http://australianmuseum.net.au/evolutionary-history-phylogenetic-diversity-pd-and-tipping-points ).

Understanding the pattern of PD loss for random species extinctions can provide a good benchmark for conservation planning. How does PD loss increase as the number of such species extinctions increases?

Faith (2008; see also Faith and Williams, 2006; Faith, 2007) proposed a power law curve for the species-PD relationship:

“PD thus defines a species–phylogenetic diversity curve that is analogous to the well-known species–area curve (see Faith & Williams, 2006) when number of species sampled is plotted against the PD value of the set.” and “PD defines a species–PD curve analogous to the well known species–area curve. A simple example of such a curve is derived here from a study by Pillon and colleagues (2006). Random taxon samples of different sizes from a given phylogenetic tree produce a roughly linear relationship in log–log space.”see figure

That evidence for that proposed power law model for a species-PD relationship was limited because the sampling of sets of taxa was constrained by membership in geographic localities. More recently, Morlon et al (2010) have provided empirical support for this proposed power law model. They calculated species–phylogenetic diversity (PD) curves for 4 different phylogenetic trees from 4 Mediterranean-type ecosystems. For each value of species richness (S), they calculated the PD (sensu Faith 1992) of 100 communities obtained by randomly sampling S species across the tips of each phylogeny. They found that the species-PD relationship is well fit by a power law for all four phylogenies.

These findings further support the use of PD in conservation planning. Morlon et al also provide support for the use of PD over altenative “phylodiveristy” measures:
“This metric is proportional to species richness for a star phylogeny (i.e. a phylogeny where species share no branch-length), rendering comparisons with the traditional species–area relationship possible. PD has the added advantage of being the phylodiversity metric of choice in conservation research”